Automatic plotter utilizing a coordinate grid device

ABSTRACT

A device for determining position coordinates of points on a surface which includes a conducting grid structure having at least two grid elements to be placed over or under a surface and a cursor structure having a circular conducting loop element to be moved across the surface of the grid structure. An alternating electric signal is supplied to either the cursor conducting loop or to each of the conducting grid elements. This signal induces a signal in each element of the unexcited conducting structure. Position coordinates are determined by apparatus which measures the induced signal or signals and records the signal change produced when the cursor is moved across the grid surface. Several embodiments of measuring devices which determine the distance between arbitrary points on a surface such as a map, graph, or photograph are illustrated. Automatic plotting embodiments are also shown and described in which the plotting motion is determined by comparing signals representing the measured loop position on the grid with a preselected set of command signals.

United States Patent 1 Bailey [54] AUTOMATIC PLOTTER UTILIZING ACOORDINATE GRID DEVICE [75] Inventor: Knight V. Bailey, Birmingham,Mich.

[73] Assignee: The Bendix Corporation, Southfield,

Mich. [22] Filed: Aug. 12, 1971 [21] Appl. No.: 171,230

Related U.S. Application Data [62] Division of Ser. No. 805,559, March10, 1969, Pat.

[52] U.S. Cl. ..318/568, 318/608, 318/653 [51] Int. Cl. ..G05b 19/42[58] Field of Search ..318/653, 162, 647, 608, 567, 318/568, 569; 346/29[56] References Cited UNITED STATES PATENTS 3,673,584 6/1972 Farrand..336/l29-X 3,376,578 4/1968 Sawyer ..346/29 3,225,337 12/1965 .lacoby318/569 X R27,289 2/1972 Sawyer 318/135 X 3,304,612 2/1967 Proctor etal.. ..346/29 X 3,343,072 9/1967 lhlenfeldt ..318/569 X OTHERPUBLICATIONS lnductosyn published by Farrand Controls Incorporated,Valhalla, N.Y., dated Sept. 1960.

[111 3,725,760 1 Apr. 3, 1973 Primary ExaminerBenjamin DobeckAttorney-Lester L. Hallacher [57] ABSTRACT A device for determiningposition coordinates of points on a surface which includes a conductinggrid structure having at least two grid elements to be placed over orunder a surface and a cursor structure having a circular conducting loopelement to be moved across the surface of the grid structure. Analternating electric signal is supplied to either the cursor conductingloop or to each of the conducting grid elements. This signal induces asignal in each element of the unexcited conducting structure. Positioncoordinates are determined by apparatus which measures the inducedsignal or signals and records the signal change produced when the cursoris moved across the grid surface. Several embodiments of measuringdevices which determine the distance between arbitrary points on asurface such as a map, graph, or photograph are illustrated. Automaticplotting embodiments are also shown and described in which the plottingmotion is determined by comparing signals representing the measured loopposition on the grid with a preselected set of command signals.

7 Claims, 20 Drawing Figures 1 agn:

HUT 5 BF 9 PAIEI'IIEUAFRIB ms BY Ma /44% AUTOMATIC PLOTTER UTILIZING ACOORDINATE GRID DEVICE CROSS-REFERENCE TO RELATED APPLICATIONS This is adivisional application of application Ser. No. 805,559, filed now US.Pat. No. 3,647,963 Mar. 10, 1969, by Knight V. Bailey, also assigned toThe Bendix Corporation.

BACKGROUND OF THE INVENTION 1. Field of the Invention A device fordetermining position coordinates of points on a surface.

2. Description of the Prior Art There are a number of purely measureddevices which attempt to rapidly and accurately provide the positioncoordinates of points on a surface to determine distances betweenpoints. One such device comprises a multiple grid structure wherein eachgrid includes sets of closely spaced, long parallel conductors. Theparallel conductors of one grid run perpendicular to the parallelconductors of the other. Measurement is made by moving a conductingprobe formed in the shape of a pencil point across the grid surface. Theprobe is energized by an alternating electric. signal which produces acapacitive coupling between the probe and the grids and thereforeinduces a voltage in the grid wires located in the near vicinity of theprobe. Electronic circuitry determines probe position by a simpleamplitude discrimination which identifies the grid wire nearest theprobe. A major objection to this device is that resolution is determinedby the spacing between parallel grid conductors and is thereforeinherently limited. Greater accuracy is achieved as the parallelconductors are moved closer together. But when an amplitudediscrimination system is used, it is necessary to maintain enoughspacing between the grid wiresto insure that definite points of maximumvoltage exist on the grid. lf the conductors are spaced too closelytogether, it will not be possible to tell which conductor is receivingthe largest induced voltage and is therefore closest to the point probe.Also, since amplitude discrimination measures the total distance betweenthe probe point and the grid wire, the measurements recorded will dependon the exact height of measuring probe above the grid as well as thedistance moved across the grid.

Another device which measures distance along one ordinate includes amovable conducting grid structure which contains one grid element whichis moved across a second, stationary grid structure containing two gridelements. The three grid elements are identical. All have equallyspaced, parallel conductive portions which are alternately connected attheir end points so that the grid elements comprise continuousconductive elements which define long, adjacent parallel loops. Theconducting grid structures are first aligned so that the parallelconductive portions of each grid element run parallel to the parallelconductive portions of the other two grid elements. The grid structuresare then placed over a surface to be measured. An alternating currentelectric signal is supplied to the movable grid structure, and thissignal induces a signal in the grids of the second grid structure. Thisdevice avoids many of the problems inherent in the previously describeddevice because the position of the movable grid structure with respectto the stationary grid structure is determined by comparing the signalsinduced in the two grid elements of the stationary grid structure witheach other. Motions such as a lifting of the movable grid structureslightly away from the stationary grid structure will not produceerroneously position measurements with this device. A lifting of themovable grid structure will simply decrease both of the induced signals.Moving the movable grid structure across the stationary grid structurewill change one induced signal with respect to the other.

The most serious limitation of this device is simply that it willmeasure distance only along one axis, that is, the axis runningperpendicular to the long, parallel conductive portions of the threegrid elements. Therefore, in order to measure the distance between thegrids two points must be positioned along the straight line connectingthe two points in question. Either the grid structures or the surfacebeing measured must therefore be moved and realigned before almost everymeasurement. This limitation, which restricts positioning determiningcapability to be along a single ordinate, clearly eliminates anypossibility for such structure to be incorporated into an automaticplotter which must be able to operate along all possible line paths.

Conventional plotters include a plotting pen attached to mechanicaldrive apparatus which moves the pen in any desired direction across aplotting surface. Pen position is determined by measuring the positionof elements of the mechanical drive apparatus. For example, in oneconventional device the pen is attached to a first lead screw assemblywhich extends over a plotting surface. This first assembly is attachedto a second lead screw assembly placed at one edge of the plottingsurface and perpendicular to the first lead screw. Pen position isdetermined by measuring the rotational position of the lead screws,which are calibrated in terms of linear position. However, since theactual position of the pencil or drawing means is not measured, errorsare introduced to such systems if the lead screws are thrown out ofalignment so that they are not orthogonal to each other or parallel tothe edges of the plotting surface, or ifthe relationship between pencilposition and the position of the drive mechanism is incorrectlycalibrated. 1

SUMMARY OF THE INVENTION This invention comprises unique conducting gridand cursor designs, which when incorporated into position determiningdevices provide output electrical signals which indicate with extremeaccuracy the position of the cursor on a grid structure. This inventionalso includes several unique apparatuses for measuring the electricalsignals which indicate cursor position. Further, this inventionencompasses complete, unique position determining devices. The positiondetermining apparatus of this invention can be embodied in a number ofdevices which include such things as measuring devices and automaticplotting devices. The apparatus of this invention includes means forproviding an excitation signal to either a conducting grid structure ora conducting cursor structure, and means for measuring a signal inducedby the excitation signal to determine cursor position on the gridstructure. Measuring devices simply transmit signals indicating cursorposition to an output display device. The illustrated plotting devicescompare output signals which represent cursor position to preselectedcommand signals which represent particular positions on the surface ofthe grid structure. The signal differences between the measured signalsand the command signals are then used to operate drive apparatus formoving the cursor to the position represented by the command signals.The illustrated embodiments show measuring devices and plotting devicesfor operating on a single surface. This invention can also be embodiedin devices such as stereoplotters.

Each embodiment shown herein includes apparatus for supplying analternating current excitation signal to either a grid or cursorstructure which induces an electrical signal in the unexcited conductingstructure. Each embodiment also includes apparatus for processing andidentifying induced signals to determine the position of the conductingcursor on the conducting grid structure.

Further, each of the embodiments shown herein of this invention includea grid structure or grid array having at least two grid elements printedon nonconductive backings. Each grid element comprises a single,continuous electric conductor that is folded or convoluted to form aplurality of equally spaced, long, parallel conductive portions that arealternately connected at their end points by shorter conductingportions. As used herein, the word convoluted" is to be interpreted inaccordance with the definition presented in Van Nostrands ScientificEncyclopedia, 4th Edition. The long, parallel conducting portions of onegrid element are placed perpendicular to the long, parallel conductingportions of the other. Each of the cursors illustrated herein to bemoved across the surface of this grid structure include at least oneconductive loop-shaped element having a transverse dimension equal to anodd multiple of the spacing between two adjacent long, parallelconducting grid portions. When an alternating current excitation signalis supplied to either the elements of the grid or cursor structures, anelectric signal whose maximum amplitude, or in other words voltage,varies sinusoidally as the cursor is moved across the surface of thegrid structure is induced in the unexcited conductive elements. Thissignal variation provides data which can be processed to provide a veryaccurate indication of cursor position. Further, an accurate measurementis obtained with this invention regardless of where the cursor isinitially placed on the surface of the grid structure, and regardless ofhow small or how great a distance the cursor is moved.

The embodiments shown herein of this invention illustrate variousdevices for measuring the change in an induced signal caused by cursormovement and therefore provide an output indication of cursor position.One embodiment of this invention shown herein illustrates amplituderatio measuring apparatus which accurately indicates the coordinateposition of a cursor on a grid structure by comparing the amplitude of asignal induced in one grid element with the amplitude of a signalinduced in an offset grid element. The amplitudes of these two signalsvary with respect to each other as the cursor is moved across thesurface of the grid structure. Other embodiments illustrate severaldifferent phase measuring constructions which measure cursor position bycomparing the phase of a summation induced signal having signalcomponents from several offset conductive grid elements with the phaseof a reference signal. The phase of the summation signal shifts as thecursor is moved across the surface of the grid structure. Eachillustrated embodiment provides an extremely accurate measurement ofcursor position. Further, each of the embodiments is constructed suchthat a slight lifting of the cursor away from the surface of the gridstructure will not cause the apparatus to provide an erroneousdetermination of coordinate position.

Visualizing the signals produced using cursor loops or probes havingdimensions other than those taught by this invention clearly indicatesthat a cursor having a single loop with a transverse dimension equal toan odd multiple of the spacing between adjacent parallel grid portionsprovides a signal which more accurately indicates cursor position thando probes or loops having other sizes. A probe having a loop dimensionedsmaller than adjacent conductor spacing will permit operation in theintended manner because a loop must have a finite dimension, and withthe parallel conductors of the grid closely spaced the loop diameterwill be appreciable with respect to such spacing. However, such a cursorwill not operate as efficiently as a probe having a dimension equal toan odd multiple of conductor spacing, because the voltage inductioncontribution with respect to each grid conductor will not be the same.

Choosing a symmetric loop with a transverse dimension equal to an evenmultiple of the spacing between two adjacent parallel conductingportions provides no net induced signal whatsoever. With a loop of suchdimensions, the signal induced in one parallel conducting grid portionwill exactly cancel the signal induced in another parallel portion.These two induced signals will cancel each other no matter where such acursor is placed on the grid. Thus, it is clearly seen that the mostmeaningful signal is provided when using a symmetric cursor loop havingits largest transverse dimension equal to an odd multiple of the spacingbetween adjacent parallel grid portions, and that as this transversedimension 'is varied from this preferred condition toward one or theother of the two extreme cases just discussed, the signal becomes muchless meaningful.

The phrase signal induced with respect to a particular conductiveelement is used herein to describe a signal induced by an excitationsignal, because with this invention an induced signal that varies inproportion to cursor displacement is provided if an excitation signal issupplied to either a grid or cursor element. Therefore, a signal inducedwith respect to" a particular cursor includes both the signal induced inthat cursor if an excitation signal is supplied to a grid element, andthe signal induced in a grid element by an excitation signal supplied tothe cursor. Signals induced with respect to a grid element of thisinvention indicate cursor displacement along an axis runningperpendicular to the long, parallel conducting sections of that gridelement. Each of the coordinate position determining devices shownherein include means for providing two induced signals indicatingdisplacement of a cursor along a grid ordinate. These two signals'areprovided to eliminate ambiguities as to the interpretation of measuredresults when only a single signal indicating displacement along onecoordinate is provided. In a number of embodiments, signals whichindicate the coordinate position of a cursor on the surface of a gridstructure are provided by a cursor having a single, circular conductingloop and a grid structure having four grids with the long, parallelconducting portions of two of the grids running parallel to the X axisof the grid structure and the long, parallel conducting portions of theother two grids running parallel to the Y axis of the grid structure.The conducting grids with long, parallel conducting sections runningparallel to each other are displaced slightly from each other so that anexcitation signal will provide different signals induced with respect toeach of the parallel grids. One embodiment of this invention providesthe desired two difference induced signals for indicating cursorposition along an ordinate by using offset cursor conducting loopsinstead of offset parallel grids.

Other novel features illustrated by the various embodiments of thisinvention include the illustration of a cursor having a single circular,conducting loop element with a diameter equal to an odd multiple of thespacing between adjacent long, parallel conducting portions. It isadvantageous to use such a cursor in many embodiments of this inventionbecause cursor rotations will not affect measurements of coordinateposition. Another embodiment illustrates a cursor having two offset,circular, conducting loops which when used with a grid structure havingfour separate grid elements provides induced signals which can beprocessed to determine both a coordinate position and the angularorientation of the cursor.

The various embodiments of this invention shown herein illustratedifferent novel elements of this invention. It is understood that anyparticular novel structure incorporated in a particular embodiment shownherein could also be incorporated in any of the other embodiments shownherein and in a great number of embodiments not shown herein. Forexample, a particular novel grid structure or signal identifyingapparatus incorporated in, say, a position measuring device in which anexcitation signal is supplied to a cursor to induce signals in theelements of the grid structure, could also be incorporated in, say, anautomatic plotting device in which excitation signals are supplied tothe elements of the grid structure to induce signals in a cursor.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects, features, andadvantages of this invention will become apparent from a considerationof the following description, the appended claims, and the accompanyingdrawings.

FIG. 1 is a schematic diagram illustrating the position determiningdevice of this invention embodied in a measuring device.

FIG. 2 is an enlarged and exploded perspective view, partly schematic,of the grid structure and circular loop cursor illustrated in FIG. 1 andin subsequent embodiments of this invention.

FIG. 3 is a further enlarged perspective view of the cursor shown inFIG. 2.

FIG. 4 is an enlarged cutaway view, partly schematic, showing two of thegrid elements included in the grid structure illustrated in FIGS. 1 and2.

FIG. 5 is an enlarged view of a portion of FIG. 4.

FIG. 6 is a graph illustrating the maximum amplitude of the signalsinduced with respect to the two grids shown in FIGS. 4 and 5 fordifferent positions of the cursor.

FIGS. 7a, 7b and 7c graphically illustrate alternating current signalsassociated with the maximum signal amplitudes illustrated in FIG. 6.FIGS. 7a, 7b and 7c illustrate one complete Hertz of two grid signalsand their summation signals for three different, specific cursorpositions.

FIG. 8 is a partial plan schematic view of an alternate grid elementdesign from the grid elements illustrated in FIGS. 1, 2, 4, and 5.

FIGS. 9a, 9b and 9c graphically illustrate the alternating currentsignals illustrated in FIG. 7 with one of the signals shifted in phaseby 90. FIGS. 9a, 9b, and 9c illustrate one complete Hertz of the phaseshifted and unshifted signals and their summation signal for the threespecific cursor positions illustrated in FIGS. 7a, 7b, and 7c.

FIG. 10 is a graph which illustrates a summed and processed inducedsignal shifted by 30 with respect to a reference signal. This phaseshift is caused by cursor displacement.

FIG. 11 is a graph which shows the induced and reference signals of FIG.10 with the reference signal shifted by the apparatus of this inventionto be in phase with the induced signal.

FIG. 12 is a schematic diagram illustrating an alternate embodiment ofthe position determining apparatus of this invention incorporated into ameasuring device in which excitation signals are supplied to the gridstructure and induced current signals are established in severalconducting loop cursors.

FIG. 13 is a schematic diagram illustrating this invention embodied in ameasuring device which contains a cursor design having two circularconducting loops so that indications of the coordinate positions and theangular orientation of the cursor are obtained.

FIG. 14 is a schematic diagram which illustrates a measuring deviceembodiment of this invention which includes two structures each of whichcompares the change in amplitude of one induced signal with the changein amplitude of another induced signal to determine the position of acursor on the surface of a grid structure.

FIG. 15 is a schematic diagram which illustrates the positiondetermining apparatus of this invention embodied in an automaticplotting device.

FIG. 16 is a schematic diagram which illustrates an automatic plottingdevice embodiment of this invention which includes a cursor havingseveral offset conducting loops so that several signals indicatingcursor position are obtained simultaneously with respect to a singlegrid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 1. Description of thePosition Measuring Apparatus of FIG. 1 Using the Detailed Relationshipsand Operative Information Provided in FIGS. 2-10 a. General DescriptionThe embodiment of FIG. 1 illustrates a measuring device 10 whichincludes signal generating apparatus 12 which transmits an alternatingcurrent excitation signal to a cursor 14 and another alternating currentsignal to phase identifying apparatus 16. The excitation signal suppliedto the cursor 14 acts to induce a plurality of signals in a gridstructure 18. These induced signals are transmitted to a signalprocessing apparatus which produces a summation signal whose phase shiftis in proportion to displacement of the cursor 14 across the surface ofthe grid structure 18. This phase shift is measured by the phaseidentifying apparatus 16 which provides an output signal indicatingcursor displacement from an arbitrarily selected reference point on thesurface of the grid structure 18.

The alternating current signal producing apparatus 12 includes a clocksignal source 20 which emits a 3 MHz alternating current squarewavesignal. This signal is sent both to the phase identifying apparatus 16and to a reference counter 22 which divides the 3MHz signal by 1,000 toprovide a BKHZ, squarewave AC signal. The 3KHz signal emitted from thecounter 22 is transmitted to a 3KHz filter 24 which combines selectedsignal overtones, filters unwanted overtones, and filters out unwantednoise signals to provide a pure sinusoidally varying 3KHZ signal. Thissignal is then amplified by a drive amplifier 26 and transmitted througha coaxial cable 28 to the movable cursor 14.

b. Grid Array and Cursor Design An enlarged view of the coaxial cable 28and cursor 14 is provided in FIG. 3. The individual conductors 30 and 32of the coaxial cable 28 divide to form a conducting, circular loopelement 34 of the cursor 14. The loop 34 includes a number of windingsso that a more intensified signal is available to induce an alternatingelectric signal in the grid elements of the grid structure 18 than wouldbe the case for a single circular winding. The circular loop 34 is heldin a molded, plastic head 36, formed, at least in the area withing thecircular loop 34, of a clear plastic, so that an operator can see thesurface over which the cursor is being moved. A crosshair pattern 38 isformed on the bottom surface of the cursor head 36 at the center of thecircular loop 34 to further assist an operator in placing the cursorprecisely over particular positions of interest on a surface.

The grid structure 18 (FIG. 2) includes four individual grid elements40, 42, 44, and 46. The grids are shown as printed circuits formed onfour identical epoxy glass backings 48. The grid elements are identical.So for illustration consider grid 40 which comprises a single, printed,continuous electric conductor that is convoluted or bent to form aplurality of equally spaced, long, parallel portions 50 which arealternately connected at their end points by the shorter conductingportions 52. The diameter of the circular conducting loop 34 included inthe cursor 14 is equal to an odd multiple of the spacings between twoadjacent long, parallel conducting grid portions 50. As used herein, theterm odd multiple includes the number one. When the cursor is movedacross the grid 40 in a direction perpendicular to the long, parallelconducting portions 50, a 3KI-Iz signal whose maximum amplitude variessinusoidally in response to cursor displacement is induced in the grid40. This direction will now be arbitrarily defined as the Y ordinate ofthe grid structure 18 and will be referred to as such hereinafter. The

graphed values labeled grid 40 voltage on FIGS. 6, 7a, 7b, and 7cillustrate this change in the maximum amplitude of the signal induced ingrid 40 as the cursor is moved along the Y coordinate of that grid. Thischange in the maximum amplitude of the induced signal can also bereferred to as the change in the induced voltage.

Since, as FIG. 6 illustrates, when the cursor 14 is moved along the Yaxis of grid 40 the induced voltage varies sinusoidally with cursordisplacement, it can be seen that these two conductive elements providean induced voltage which more accurately represents cursor position thanhas previously been obtained with other grid and cursor designs. Note,however, that the single grid 40 voltage illustrated in FIG. 6, which isproduced using the single loop cursor 14 and the one grid 40, does notprovide a completely unambiguous indication of cursor position. For eachpoint on the rising slope of the grid 40 voltage curve, there exists apoint having an equal amplitude and carrier polarity on the fallingslope of that curve. Therefore, a second grid 42, referred to herein asa quadrature grid, which runs parallel to the grid 40 and is placeddirectly below it is included in the grid structure 18 to assist inproviding a completely unambiguous measurement of cursor position. Thegrid 42 is similar to the grid 40 and also comprises a single, printed,continuous electric conductor having a plurality of equally spaced,long, parallel portions 54 which are alternately connected at their endpoints by the shorter conducting portions 56. The long, parallelconducting portions 54 of grid 42 are equal in length to and runparallel to the long conducting portions 50 of grid 40. Further, thespacing between the conducting portions 54 is equal to the spacingbetween the conducting portions 50 of grid 40. However, as can be bestseen in FIG. 4, the printed circuit structure of grid 42 is shifted withrespect to the grid 40 so that each of the parallel conductors 54 aredisplaced a preselected distance in the Y direction from the conductors50 of grid 40. In the embodiment shown in FIG. 4, the conductingsections 54 of grid 42 fall halfway between each of the conductingsections 50 of grid 40. Thus, when a circular conduction loop is placedover the grid structure 18 and excited with an AC signal, signals havingtwo different maximum amplitudes will be induced in the grids 40 and 42.Note the grids 40 and 42 provide only cursor coordinate position alongthe Y axis of grid structure 18. The grids 44 and 46, which appeardirectly below the grids 40 and 42 and and run perpendicular to thosegrids provide coordinate position along the X axis of grid array 34. Thegrids 44 and 46 are identical to the grids 42 and 44 and are arrangedwith respect to each other as are the grids 40 and 42. That is, thelong, parallel conductive portions 58 of grid 44 appear directly aboveand midway between the long, parallel conductive portions 60 of grid 46.Therefore, to avoid repetition, no detailed description of those gridswill be provided. Similarly, the signal processing apparatus'20 andphase identification apparatus 16 for receiving signals from the grids44 and 46 and determining X coordinate cursor position is also identicalto the apparatuses 20 and 16 shown for receiving signals from grids 40and 42 and will not be described in detail.

An understanding of the manner in which the signals induced in grids 40and 42 change as the cursor 14 is moved along the Y axis of gridstructure 18 is provided by viewing FIGS. 5, 6, and 7. The points a, b,and c designated on FIG. 5 indicate the position of the cursor 14 whenthe cursor cross-hair pattern 38 is placed directly above one of thosepoints. The maximum am plitudes of the signals induced in the grids 40and 42 when the cursor 14 is placed at one of those points are indicatedwith the letters a, b, and respectively on FIG. 6. The induced signalsthemselves and their summation signal produced for the three cursorpositions a, b and c are graphed in FIGS. 70, 7b, and 7c, respectively.Note that FIGS. and 6 show that the induced signals vary through onecomplete maximum signal amplitude cycle as the cursor is moved adistance equal to twice the spacing between adjacent long, parallel conductive portions 50. FIG. 5 shows point b displaced from point a adistance along the Y axis of grid 40 equal to one-third the distancebetween point a and the next adjacent parallel conducting portion 50.FIG. 6 shows point b displaced from point a a distance of 30 oronetwelfth of a cycle along the sinusoidally varying graphed values ofFIG. 6. Similarly, point c is displaced from point b a distance equal toone-half the spacing between adjacent parallel grid conducting portions50 shown on FIG. 5, and is displaced one-fourth cycle from point b onthe graph of FIG. 6. The signals induced in the grid 40 and 42 aretherefore represented by the mathematical equations:

E =E sin (y/d X 360) sin wt and E E cos (y/d X 360) sin wt where E themaximum amplitude of the induced signal value that can be obtained froma signal grid using a given excitation potential. This amplitude isillustrated at point a on FIG. 5. y linear displacement along the Y axisof grid structure 18.

d twice the distance between adjacent, long, parallel grid conductingportions of a grid.

to frequency (3KHz in this embodiment).

t time in seconds.

Note that point b, as well as being displaced from point a along the Yaxis of grid 40, is also displaced from point a a distance along the Xaxis of that grid. This lateral displacement will not be indicated inany way by a change in the induced signal measured across the leads 62and 64 (FIG. 4) to the grids 40 and 42. Only the component of motion inadirection perpendicular to the long conducting sections 50 and 54 willproduce a change in the signal induced with respect to these two grids.Since each of the grids forming the grid structure 18 are designed tomeasure position only along one axis, the induced signals caused byelectrical coupling between the short, connecting portions 52 and 56 ofagrid and the cursor, produced when the cursor nears those portions, mustbe accounted for. It can be seen from either FIG. 3 or 4 that thevoltage induced in, say, the connecting portions 52 of grid 40 does notindicate the position of a movable cursor along the Y axis. If a cursoris moved along the X axis of grid structure 18 while keeping itsposition with respect to the Y axis constant, the signal measured acrossthe leads 62 of grid 40 will be slightly larger when the cursor is'neara connecting portion than when it is near the center of the grid. Sinceany change in the signal coming from grid 40 is interpreted asindicating motion along the Y axis, if the signal induced in theconnecting portion 52 were allowed to reach the phase identifyingapparatus 16, errors would be introduced into the position measurementsprovided. Therefore, the encircling conducting section 66 is included aspart of each of the printed grids forming the grid structure 18 for thepurpose of providing a signal to cancel the signal induced in theportions 52. The encircling section 66 runs parallel to and close to theconnecting portions 52 so that when a cursor is placed near a connectingportion 52 and induces a signal in that section, an electric signal willalso be induced in the encircling conductor 66. Note that the twosignals induced in the conducting sections 52 and 66 will besubstantially equal and electrically opposed to each other, therebycancelling each other so that no net electric signal is provided in thegrid which can be measured across the leads 62.

FIG. 8 illustrates an alternate grid winding designed to negate theeffect of induced error signal provided by the coupling between thecursor and the shorter connecting portions of a grid. The grid element68 shown in FIG. 8 is similar to the grids comprising the grid structure18 in that it is formed from a single, printed, continuous electricalconductor that has long, parallel conducting portions 70 connected attheir alternate end points by the shorter conduction portions 72.However, unlike the grids forming the grid structure 18, the continuousconductor forming the grid 68 is folded back along itself so that long,parallel conducting portions 74 run parallel to and are placed close tothe conducting portions 70. Further, the conducting portions 74 areconnected at their end points by shorter conducting portions 76 whichrun parallel to and are spaced between the end portions 72. As thecursor 14 is I moved near the connecting portions 72 of the grid 68, a

signal will be induced with respect to those portions. If the portions76 did not exist, the signals induced with respect to the portions 72would cause an error indication to be read across leads 78 of this grid.Note, however, that grid 68 is constructed such that, when a signal isinduced in a connecting portion 72, there will be a connecting portion76 close enough to that portion so that there will also be a signalinduced in a portion 76. The two signals induced in the two connectingportions are equal and electrically opposed to each other and willtherefore cancel. Also, if an excitation current is supplied to the grid68 rather than to the cursor 14, the excitation current traveling in aconducting portion 72 will be opposed by the excitation signal travelingin an adjacent portion 76. There will therefore be no net currentinduced in the cursor caused by an electrical coupling with theconnecting edge portions of the grid 68.

everything else being equal, than can be provided with I a grid such asgrid 40.

c. Signal Processing & Phase Identification Apparatus FIGS. 6 and 7indicate that the maximum amplitudes of the signals induced in grids 40and 42 by the excitation signal supplied to the cursor l4 vary as thecursor is moved along the Y axis of grid structure 18. However, thephase of the induced signals does not change in a manner whichaccurately indicates cursor displacement. Note that FIG. 7 shows thatthe signals induced in grid element 40 and 42 and their summation signalare always either perfectly in phase with each other, or that one of thesignals will be exactly 180 out of phase with the other two. Theposition determining device 10 (FIG. 1) therefore includes a signalprocessing apparatus which receives the induced signals from grids 40and 42, and produces a signal whose phase shift is in proportion tocursor displacement. The signals from the grids 40 and 42 are firstamplified by gain amplifiers 80 and 82, respectively, so that strongerand therefore easier signals to work with are obtained. A phase shiftingapparatus 84 then shifts the phase of the signal from the quadraturegrid 42 .by 90, or one quarter cycle. This phase shift does not changethe induced voltage values. The manner in which the induced voltagechanges as the cursor 14 is moved across the surface of the gridstructure 18 is still as illustrated by FIG. 6. However, the phaserelationship of the two induced alternating current signals to eachother is changed. This relationship for the three cursor positions 11,b, and c is shown by the graphs of FIGS. 9a, 9b, and 9c, respectively.The unshifted signal from grid 40 and the 90 phase shifted signal fromgrid 42 are then summed in the summation amplifier 86. FIG. 9 alsoillustrates the summation induced alternating current signal produced bythe summation amplifier 86 for the three cursor positions indicated.

FIG. 9a shows the waveform with the cursor 14 at point a of FIG. 5.There will still be no net signal induced in the quadrature grid 42because the center point of cursor 14 is directly over one of conductors54. Therefore, the summation signal produced by summation amplifier 86will simply equal the signal coming from the amplifier 80 when thecursor is at point a. FIG. 9b illustrates the induced grid 40 signal,and the quadrature grid 42 signal, both of which are summed by amplifier74, and the summation signal provided by the summation amplifier whenthe cursor is at point b on the grid structure 18. Because the phase ofthe signal coming from the quadrature grid 42 has been shifted 90 withrespect to the signal from the grid 40, the summation signal produced bythe summation amplifier 86 when the cursor is at point b is shifted byonetwelfth of a cycle or 30, from the summation signal illustrated inFIG. 9a. Note, as was the case for the signals shown in FIG. 7, themaximum amplitudes of the alternating current signals from the grid 40and the quadrature grid 42 vary in accordance with the changes in cursorposition. Note, however, than even though the maximum amplitude of thesetwo signals changes, the maximum amplitude of the summation signalillustrated in FIG. 9b has not changed from that shown in FIG. 9a. Onlythe phase of that signal has been shifted.

FIG. 90 illustrates the grid 40, quadrature grid 42, and summationsignals produced when the cursor is at position c. Note that, as was thecase previously, the

E =14 E sin (y/d X 360") sin t+A E cos (y/d X 360) cos wt where:

an amplification factor provided by the processing apparatus (20) andThe remaining symbols are as previously defined. Manipulating the abovein a straightforward mathematical fashion produces:

sum Esin (Y/d 360+mt) Thus, this mathematical expression confirms theillustration of FIG. 9 which shows that the signal leaving the summationamplifier 86 is an alternating current signal whose phase shiftslinearly and in direct proportion to any displacement of the cursoralong the Y axis of grid structure 18.

This summation signal is filtered by a SKI-I2 frequency filter 88 whichremoves unwanted noise signals and overtones from the summation signaland provides a pure sine wave signal for further processing. A zerocross-over detector 90 detects the node or zero signal value points ofthis sinusoidally varying summation signal and amplifies said signal,thereby converting the sinusoidally varying summation signal shown inFIG. 8 to the summation squarewave signal shown in FIGS. 10 and 11. Thissummation squarewave signal is transmitted to the phase identifyingapparatus 16 which provides an output signal indicating cursor positionby measuring the phase change of this summation squarewave signalproduced when the cursor 14 is moved along the Y ordinate of gridstructure 18.

The phase identifying apparatus 16 includes phase comparator logic 92, adevice well known to those skilled in the art, which receives thesummation squarewave signal from the zero cross-over detector 90 andcompares the phase of that signal to the phase of a reference signal.This reference signal is a 3KI-Iz squarewave signal which is produced bythe clock source 20, a switching logic 94 and-a counter 96. Clock source20 emits a 3MI-Iz square-wave signal which, when the reference andsummation signals coming to the comparator logic 92 are in phase witheach other, is transmitted through switching logic 94 and over line 98to the counter 96. The counter 96 is a device well known to thoseskilled in the art and includes a series of switching circuits. Thecounter is constructed to provide an output signal of fixed amplitudewhose polarity shifts only in response to action of said switchingcircuits. These switching circuits are responsive to the incoming 3MHzsignal and are constructed such that they switch the polarity of theoutput signal of the counter 96 whenever 500 input signal pulses arereceived over line 98. Counter 96 therefore transmits a 3KHz squarewavereference signal to the phase comparator logic 92. The phase comparatorlogic 92 compares the phase of this reference signal with the summationsquarewave signal transmitted from detector 90. When the phasecomparator logic 92 determines that these two signals are out of phasewith each other, it.transmits a signal to the switching logic 94 whichalters the manner in which signals are transmitted to the counter 96 andthereby shifts the phase of the reference signal being supplied to thephase comparator logic 92.

Suppose for example, that the phase comparator logic 92 detects a phaserelationship such as that shown in FIG. in which the squarewavesummation signal leads the squarewave reference signal by 30. The phasecomparator logic 92 would then direct the switching logic 94 to transmitone of the signal pulses from clock source 20 to counter 96 by way ofline 100. This pulse would therefore bypass one of the switching circuitelements contained in the counter 96 and cause the polarity of thecounter output signal to be shifted after only 499 pulses are receivedfrom the clock source 20. This advances the phase of the referencesignal by l/ 1,000 of a Hertz toward the summation signal. Thisadvancement procedure will be repeated for every pulse emitted by thecounter 96 for as long as the phase comparator logic 92 detects thesummation signal leading the reference signal.

The entirely in-phase condition for the reference and summation signalsis shown in FIG. 11. As can be seen by that figure, the reference signalhas been shifted to the position occupied by the summation signal inboth FIGS. 10 and 11. Thus, the reference signal has been shifted by 30or one-twelfth a cycle. Similarly, if the phase comparator logic 92detects the summation signal is lagging the reference signal, it directsswitching logic 94 to stop transmitting signal pulses from the clock 20to the counter 96 until the summation and reference signals are in phasewith each other. Note that whenever one pulse is emitted by the clocksource 20 which does not reach counter 96 the phase of the referencesignal coming to the phase comparator logic 92 will be retarded by l/l,000 of a cycle.

In the above example, the phase of the reference signal was shiftedthrough 30 to be in phase with the summation signal. This illustrationwas chosen to aid understanding of the phase comparator logic 92,switching apparatus 94, and counter 96. In actual operation, thesedevices operate with such speed that the reference and summation signalscoming to the phase comparator logic 92 will be substantially in phasewith each other at all times no matter how quickly the cursor 14 ismoved across the surface of the grid structure 18 and no signaldifference as large as 30 will ever actually exist.

When the phase comparator logic 92 directs the switching logic 94 toeither advance or retard the phase of the signal coming from the counter96, it also directs a switching logic 102 to transmit electric signalpulses to a count storage register 104. These signal pulses act tochange the count stored in that register and therefore cause that countto be an accurate record of net cursor displacement from a referencepoint along the Y axis of grid structure 18. The phase identificationapparatus 16 is constructed such that, when the switching logic 94 andcounter 96 operate to advance the phase of the reference signal by l/l,000 of a cycle, switching logic 102 transmits one negative pulse toregister 104 which decreases the count in that register by one.Similarly, when the switching logic 94 and counter 96 operates to retardthe phase of the reference signal coming from counter 96 by l/l,000 of acycle, the switching logic 102 transmits one positive electric pulse toregister 104 which increases the count in that register by one. Thecount stored in register 104 is therefore the net number of positive ornegative pulses or phase increments that have been needed to keep thesummation and reference signals in phase with each other. The countstored in register 104 is supplied to the conversion apparatus 106 whichconverts the count stored in register 104 to a decimal indication ofcursor displacement on the surface of the grid structure 18. Since thecomparator logic 92, switching logic 94, and counter 96 act tocontinually maintain the summation and reference signals in phasewitheach other, virtually any number smaller than the numberrepresenting the phase shift produced by moving the cursor completelyacross the grid structure 18 may appear in counter 104. This count isnot limited by, say, the number of signal pulses necessary to produce acomplete one cycle phase shift. For example, suppose a count of 3,100 isstored in the register 104. As has already been stated, a count of 1,000indicates a full cycle phase shift which is provided by moving thecursor a distance equal to twice the spacing between adjacent long,parallel conducting grip portions. If the grids forming the gridstructure 18 are constructed so that these parallel conducting portionsare placed one half inch apart, the conversion apparatus 106 wouldconvert a count of 3,100 coming from the register 104 to a decimalnumber so that output display 108 would indicate a cursor displacementof three and one-tenths inches. A negative count indicates displacementin one direction while a positive count indicates displacement in anopposite direction from a reference point along the Y axis of gridstructure 18. Also note that the count stored in register 104 indicatescursor displacement with an accuracy equal to H500 of the spacingbetween two adjacent parallel conducting grid portions.

FIG. 1 shows separate output displays for indicating displacement alongthe X and Y axis of grid structure 18. This dual display arrangementprovides a record of both the magnitude and direction of cursordisplacement from a reference point. If desired, a single numberindicating the straight line distance between a given point and areference point can also be provided. The straight line distance betweena point and a reference point would simply be the hypotenuse of theright triangle having two sides equal to the displacements along the Xand Y axis of grid structure 18 illustrated in FIG. 1. Or, as anadditional option that might be accomplished using the apparatus shownin FIG. 1, the signals from the storage register 104 could be sentdirectly to a computer for further processing rather than to visualoutput display apparatus.

The operation performed by the phase comparator logic 92, switchinglogic 94 and the counter 96 when reacting to a cursor displacement isbeing described consistently herein using the term phase shift." It isrealized that the phase and frequency of any alternating current signalare so interrelated that the operation being performed could also bedescribed using the term frequency shift. Whenever the phase of onesignal is shifted relative to the other, the frequency of the signalbeing shifted is altered during the time interval during which the phaseshift occurs. Admittedly, the physical operation being performed couldbe adequately described referring to either a frequency shift or a phaseshift." The term frequency shift is not being used because it is felt itmight have suggested to some that distance between a reference point anda point of interest would also be indicated during the time intervalduring which one signal was actually being shifted with respect to theother. As can be seen from FIG. 1, this is not the case. When the cursorremains motionless over a point of interest, the counter 96 simply emitsone pulse for every 1,000 pulses received from the clock and thereference signal remains in phase with the summation signal coming fromgrid structure 18. The count register 104 will simply remain stored inthat register and will not be increased or decreased while the cursor isheld over thepoint of interest. The output display 108 will thereforeindicate the related ordinate distance between the point of interest andthe reference point as long as the cursor 14 is held over the point ofinterest.

In operation of the measuring device 10, an operator places the gridstructure 18 over or under a surface to be measured. Since theconducting grids can be printed on very thin epoxy, glass, or plasticbackings, the grid structure can be made quite flexible so thatmeasurements need not be restricted to flat surfaces. The operator thenactivates the excitation or reference signal supplied to the cursor l4and phase identification apparatus 16. The operator need not go throughany long process of precisely aligning the grid structure 18 withwhatever surface he wished to measure because the apparatus 10 isconstructed such that any point on the grid structure surface can beselected as a reference point from which measurements are to be made. Toselect a point as a reference point, the operator simply places thecursor 14 directly over that point and activates a count clear or resetswitch device 110 which erases the count in the coordinate registers104. As long as the cursor 14 is not moved from this now selectedreference point, a zero indication will remain in the count registers104, and no displacement will be indicated by the output displays 108.The operator then moves cursor 14 so that the cross-hair pattern 38appears directly over a first point of interest. As the cursor is movedacross the grid structure surface, the phase of the summation inducedsignal shifts with respect to the reference signal. The phase comparatorlogic 92 along with the switching logic 94 and counter 96 act to shiftthe phase of the reference signal and keep the reference and summationsignals continually in phase with each other. The phase comparator logic92 in combination with the switching logic 94 also acts to keep a recordof the phase shift of these two signals in a count register 104. Thiscount is displayed by the display apparatus 108 as a decimal numberindicating cursor displacement from a reference point on the gridstructure 18.

Note that the specific path followed by the cursor 14 in moving from onepoint to another will not affect the distance measurement providedbetween these two points. The direction in which the phase of the summation signal shifts with respect to the reference signal depends on thedirection the cursor is moved across the grid structure surface. Supposethe cursor is first moved in one direction so that the count register104 will be increased in the positive direction. If the cursor is movedin the opposite direction, the count in register 104 will be decreased.Suppose the cursor 14 is moved from a reference point beyond a point theoperator considers to,be of interest and then back to that point. Thecount held by the register 104 when the cursor is directly over a pointof interest will indicate the precise distance between the referencepoint and that point. In'

moving beyond the second point the count in register 104 will have beenincreased, but in moving back to that point, the count will have beendecreased. Thus, extreme convenience of operation is provided. Anoperator can select a reference point, move the cursor to be directlyover a point of interest following any path he chooses, and he will beprovided with a display of the distance between the reference point andthe point of interest. If he then desires to know the distance betweenhis selected reference point and another point of interest, he simplymoves the cursor from his first point of interest to the second point ofinterest. The output display 108 will indicate the distance between thereference point and this second point of interest. Further, if anoperator desires to change his reference point after having made anumber of measurements, he need only place the cursor over this newlyselected point he wishes to use as a reference point and activate thecount clear device 110 which erases the count in registers 104. Anyfurther shift in the phase of the summation signal caused by cursordisplacement will cause either a positive or negative increase in thecount held in a register 104. The count stored in those registers willtherefore indicate cursor displacement from this newly selectedreference point.

2. Alternate Embodiments in Measuring Devices Employing PhaseIdentification Apparatus FIG. 12 illustrates a measuring device 110embodiment of this invention in which excitation signals are supplied tothe grid structure 18 rather than to the cursor 14 as they are in theembodiment of FIG. 1. The embodiment shown in FIG. 12 also illustratesalternate signal phase identification apparatus 112 from thatillustrated in FIG. 1. Further, FIG. 12 shows that with this inventionseveral identical cursors can operate independent of each other on asingle grid.

The measuring device 110 shown in FIG. 12 includes the signal source 114which transmits 3KHz sinusoidally varying signals to the Y coordinategrid 40 and quadrature grid 42 and the signal source 1 16 whichtransmits 4KHz sinusoidally varying alternating current signals to the Xcoordinate grid 44 and quadrature grid 46 of the grid structure 18. FIG.12 shows the grid structure 18 generally and does not illustrate thefour grids 40, 42, 44, and 46 because those grids were shown in detailand fully described in FIGS. 2 and 4. Each of the four signals suppliedto the grid structure 18 acts to induce a signal in each of theillustrated cursors 14. The cursors act as electrical summers andtransmit a single summed signal having signal components introduced byeach of the four grid excitation signals to a signal phaseidentification apparatus 112.

FIG. 12 shows the apparatus 112 in detail for determining the positionof only a single cursor. The apparatus for determining the positions ofthe other cursors is identical to that shown. Further, the signalinduced in one cursor and the motion of one cursor will not affectmeasurements made for the position of another cursor. The phaseidentification apparatus 112 includes the apparatus 118 for determiningthe Y coordinate position of cursor 14 and apparatus 120 for determiningthe X coordinate position of cursor 14.

The two signal sources 114 and 116 are each similar to the signalproducing apparatus 12 illustrated in FIG. 1. They are constructed,however, to produce signals having different signal characteristics.That is, source 114 transmits 3KHz signals to the Y coordinate grid ofgrid structure 18, and source 116 transmits 4KHz signals to the X axisof grids of that structure. Therefore, the summation signal induced in acursor 14 can be separated into a first signal indicating displacementalong the X axis of grid structure 18. FIG. 9 illustrates that the phaseof a summation signal, provided by adding a first signal componentinduced with respect to a grid element and a second signal component,phase shifted with respect to the first signal component, and inducedwith respect to a quadrature grid element, will shift in proportion tocursor displacement in a direction across the long parallel conductiveportions of the two grids. The cursor 14 acts as an electrical signalsummer. The position determining device 110 therefore includes apparatusfor shifting the phase of the excitation signals supplied to the twoquadrature grids 42 and 48 instead of including apparatus for shiftingthe signals induced in the quadrature grids as shown inFIG. 1. Thesignal phase shift devices 122 and 124 which perform this phase shiftare each similar to the phase shift apparatus 84 shown in FIG. 1.

The summation signal induced in cursor 14 is expressed by themathematical equation:

E,,,,,, E sin ([(y/d) 360 E sin [(x/d) 360 e l where:

x linear cursor displacement along the X coordinate of grid structure18. w, =frequency of signal supplied to the Y coordinate grids 40 and 42(3KHz in this embodiment). m frequency of signal supplied to the Xcoordinate grids 40 and 42 (4KI-Iz in this embodiment). The remainingsymbols are defined previously. This signal is transmitted by a coaxialcable 126 through cable branch 128 to the BKHZ bandpass filter 130 andthrough cable branch 132 to the 4KHz bandpass filter 134. Bandpassfilter 130 filters out the 4KHz signal components which indicate Xcoordinate position of cursor 14 and transmits a 3KHz summation signalsuch as the signal illustrated in FIG. 9 which indicates the Ycoordinate cursor position through a gain amplifier 135 to a phasesensitive demodulator 136. The demodulator 136 also receives a 3KI-Izsquarewave reference signal. This reference signal is provided bythesignal source 114 which transmits a 3MHz squarewave signal to a counter138 which is similar to the counter 96 illustrated in FIG. 1 andoperates to reduce this 3MHz signal by a factor of 1,000 to provide the3KHz reference signal for the demodulator 136. As

FIG. 9 indicates, the phase of the summation signal transmitted to thedemodulator 136 is determined by cursor position. The phase relationshipbetween the summation and reference signals coming to the demodulator136 determines whether or not there will be a demodulator output signal.The demodulator is so designed that there will be no demodulator outputsignal if the reference signal is exactly 90 out of phase with thesummation signal.

Cursor motion along the Y axis of grid structure 18 shifts the phase ofthe summation signal and therefore provides a demodulator output signal.The demodulator output signal is transmitted to a voltage controlledoscillator 140. This oscillator responds to the demodulator outputsignal by transmitting signal pulses to the counter which act to shiftthe phase of the reference signal being transmitted to the demodulatorand therefore maintain the reference signal 90 out of phase with thesummation signal. The rate at which the voltage controlled oscillatoremits signal pulses is determined by the magnitude of the demodulatordirect current output signal. If the cursor is moved along the Y axis ofgrid structure 18 in a direction to cause a positive signal output fromthe demodulator 136, the voltage controlled oscillator 140 transmitssignal pulses to the counter 138 which also receives a positive signalover line 141 through branch 142 from the demodulator 136. This positivesignal causes the counter 138 to add pulses from the oscillator 140 tothe pulses received from the clock source 114. These signal pulsestransmitted by the voltage controlled oscillator 140 act to advance thephase of the reference signal because the counter 138 will have received500 pulses and therefore reverse the polarity of its output signal eventhough the source 114 will not have emitted 500 pulses. Similarly, whenthe cursor 14 is moved in a direction along the Y axis of grid structure18 to cause a negative value demodulator signal output, the signaltransmitted to the counter 138 over branch 142 will direct that counterto subtract the signal pulse emitted by the voltage controlledoscillator 140 from those received from the clock source 114. Thesepulses therefore retard the reference signal coming to the demodulator136.

The voltage controlled oscillator 140 transmits signal pulses to thecount register 143 as well as to the register 138. Count register 143also receives the demodulator output signal from line 141. When apositive signal is transmitted over line 141, each oscillator pulse actsto increase the count stored in that register by one, and when anegative signal is transmitted over line 141, each oscillator pulse actsto decrease that count by one.

Thus, as was the case for the count register 104 illustrated in FIG. 1,there is stored in count register 143 a record of both the magnitude anddirection of cursor displacement from a reference point along the Y axisof grid structure 18. Since adding or subtracting 1,000

' the apparatus 118, the only difference being that the apparatusdetermines coordinate position along the X axis of grid structure 18instead of the Y axis and is therefore sensitive to 41(Hz signalsinstead of 31(1-12 signals. A signal induced in one of the cursors 14 istransmitted to its 4KHz bandpass filter 134 which filters out unwantedsignal frequency components, noise signal components, and overtones andtransmits a 4KHZ, sinusoidally varying summation signal through a gainamplifier 143 to a phase sensitive demodulator 144. The demodulator alsoreceives a 4KHz, squarewave reference signal from the source 116 by wayof the counter 146. As was the case for the demodulator 136, thedemodulator 144 emits an output signal when the summation and referencesignals are not 90 out of phase with each other. This signal istransmitted to a voltage controlled oscillator 148 which acts to shiftthe phase of the signal coming from the counter 146 and therebymaintains a 90 phase relationship between the reference signal and theinduced 41(Hz summation signal as the cursor is moved along the X axisof grid structure 18. The oscillator 148 also changes the count inregister 150 as it .shifts the phase of the reference signal coming fromthe counter 146. The number stored in the register 150 thereforeindicates the magnitude and direction of cursor displacement from areference point along the X axis of grid structure 18, just as register143 records cursor displacement along the Y axis.

Operation of the apparatus 110 illustrated in FIG. 12 is similar tooperation of the apparatus illustrated in FIG. 1. An operator firstactivates the signal sources 114 and 116 to supply excitation signals tothe grid structure 18 and to the phase identifying apparatus 112. Hethen selects a reference point for a particular cursor 14 by placingthat cursor over the point he wishes to use as a reference point andactivates count clear apparatus 152, which may simply be a reset pushbutton switch, and erases the'count stored in the registers 143 and 150.As the count 14 is displaced from this selected reference point, thephase of the induced signals transmitted to the demodulators 136 and 144will shift with respect to the squarewave reference signals supplied tothose demodulators. This phase shift produces voltage outputs from thedemodulator 136 and 144 which activate the voltage controlledoscillators 140 and 148 to shift the phase of the squarewave referencesignals being supplied to those demodulators and to record these phaseshifts in the count registers 143 and 150 respectively. Thus, thenumbers stored in the registers 143 and 150 indicate cursor displacementfrom the selected reference point along the Y and X axes respectively ofthe grid structure 18.

FIG. 13 illustrates a measuring device embodiment 154 of this inventionwhich includes a unique, two-loop cursor 156 which enables themeasurement of both coordinate position and angular orientation of thecursor. The cursor 156 thus facilitates the rapid determination of boththe distance between objects on a surface such as a map placed over thegrid structure 18 and the angular orientation of objects on thatsurface. The cursor 156 includes a transparent, rectangular shapehousing member 158 which contains two circular conducting loops 160 and162. As was the case for oursor 14, the conducting loops 160 and 162 areeach of a diameter equal to an odd multiple of the spacing betweenadjacent long, parallel conducting grid portions 50. The centers of thetwo loops and 162 are separated by a distance s as illustrated in FIG.13, and a reference cross-hair pattern 164 is located midway along lines. As illustrated, the coordinate displacement positions of the centerof loop 160 are designated X,Y,, and the coordinate displacementpositions of the center of loop 162 are designated X Y The quantity (Y,Y )/2 indicates the displacement along the Y axis of grid structure 18of the cursor cross-hair 164 from a reference point, and the quantity(X, X )/2 indicates the displacement along the Y axis of grid structure18 of the cursor cross-hair 164 from a reference point. Both thequantities (Y, Y and (X, X are measures of the angular orientation, orin other words the angular displacement from a preselected referenceposition, of cursor 156 on the surface of the grid structure 18. As F1G. 13 illustrates,

the length s forms the hypotenuse of the right triangle formed with afirst side extending from the center of loop 160 along the Y axis ofgrid structure 18, and with a second side extending from the center ofloop 162 along the X axis of grid structure 18. Note, sin 0= (Y, Y )/sand cos 0 (X, X )/s. If s is chosen of unit length, (Y, Y sin 9 and (X,X cos 6.

The apparatus 154 which provides the abovedescribed measurements ofcursor coordinate position and angular orientation includes thealternating current signal source 166 which supplies a 3KHz sinusoidallyvarying alternating current excitation signal to cursor loop 160 and a4KHz sinusoidally varying alternating current excitation signal tocursor loop 162. Each of these excitation signals acts to induce asignal in each of the grids comprising the grid structure 18. Theseinduced signals are transmitted to a signal processing and phaseidentification apparatus 168 which provides output signals indicatingcursor position and orientation. The apparatus 168 is shown in detailfor determining Y coordinate cursor position and the quantity sin 0. Theapparatus for determining X coordinate position and the quantity cos 0is identical to that shown.

The signal processing and phase identification apparatus 168 includessignal processing apparatus 170 and phase identifying apparatus 118 fordetermining the Y coordinate position of loop 160 which is thereforeresponsive to 3KHz signals; and signal processing apparatus 172 andphase identifying apparatus 120 for determining the Y coordinateposition of loop 162 which is therefore responsive to 41(Hz signals.Signals from both the Y axis of grid 40 and the Y axis quadrature grid42 are transmitted to both structures 170 and 172. Induced signals aretransmitted from grid 40 by coaxial cable 174 through cable branch 176to the 3KHz bandpass filter 178 included in the apparatus 170, andthrough cable branch 180 to the 4KHz bandpass filter 182 included in theapparatus 172. Signals induced in the Y axisquadrature grid 42 aretransmitted by cable 184 through cable branch 186 to a 310-12 bandpassfilter 188 and through cable branch 190 to a 4KHz bandpass filter 192.The bandpass filters 178, 182, 188, and 192 filter out unwanted frequentcomponents, noise signals, and signal overtones to provide sinusoidallyvarying alternating current induced signals of the desired frequency forfurther processing. With regard to the apparatus 170, signals from the.bandpass filter 178 are amplified by a gain amplifier

1. An automatic plotting system comprising: a grid structure, said gridstructure including a first continuous conductive grid element having afirst plurality of equally spaced parallel portions arranged such thatcurrents in all adjacent conductors flow in opposite directions, saidfirst grid element being parallel to said surface and defining a firstcoordinate on said surface; said grid structure also including a secondcontinuous grid element having a second plurality of equally spacedparallel portions perpendicular to said first plurality of parallelportions arranged such that currents in all adjacent conductors flow inopposite directions, said second grid element being parallel to saidsurface and defining a second coordinate on said surface; a cursorstructure having at least one conductive element; means for energizingone of said structures to establish voltage induction between theconductive elements of said structures so that relative movement betweensaid structures induces voltages in the conductive elements of the otherof said structures, and utilization means responsive to said voltagesfor utilizing said voltages as output signals indicative of the positionof said cursor relative to said reference position; said cursorstructure being dimensioned with respect to said spacing of saidparallel portions such that a varying signal is induced in at least oneof said conductive elements because of said induction between saidcursor structure and said grid structure; means responsive to saidvariable signal as said cursor device is moved across said gridstructure to provide coordinate output signals indicating cursordisplacement along said grid structure; means for providing commandsignals; means for comparing said coordinate output signals to saidcommand signals to produce an error signal representative of thedifference between said coordinate output signals and said commandsignals; and means responsive to said error signal for moving saidcursor across said grid structure surface.
 2. The system of claim 1further including means for converting said variable signal into digitalposition signals; and wherein said command signals are digital commandsignals representative of a desired position of said cursor.
 3. Thesystem of claim 2 wherein said means for comparing is a digitalcomparison means responsive to said digital position signals and saiddigital command signals so that said error signal is a digital signal.4. The system of claim 3 wherein said means responsive to said errorsignal includes a digital to analog converter for converting saiddigital error signal to an analog drive signal; and further includingcursor drive means responsive to said analog drive signal.
 5. The systemof claim 4 wherein said means for providing command signals is a digitalcomputer for storing desired positions of said cursor as said digitalcommand signals.
 6. The system of claim 2 wherein said digital positionsignals are separated into X axis signals and Y axis signals; whereinsaid means for comparing includes an X axis digital comparison means anda Y axis digital comparison means; and wherein said command signals aredigital command signals, said digital comparison means being responsiveto the respective axis signals and said digital command signals torespectively generate an X error signal and a Y axis error signal. 7.The system of claim 6 wherein said means responsive to said error signalincludes an X axis digital-to-analog converter for converting said Xaxis error signal to an X axis analog drive signal, and a Y axisdigital-to-analog converter for converting said Y axis error signal to aY axis analog drive signal; and further including X axis cursor drivemeans and Y axis cursor drive means respectively responsive to said Xand Y analog drive signals for driving said cursor to a commandedposition.